Increasing demand for wireless services has led to the allocation of increasing amounts of wireless spectrum for use by wireless communications systems, such as point-to-point and point to multi-point broadband radio communications systems. In some cases, bands within the wireless spectrum that were previously allocated for another use have been allocated for use by broadband radio communications systems, as either a licensed or unlicensed band. In particular some bands, for example bands at approximately 5.4 and 5.8 GHz, were formerly allocated exclusively for use by military radar systems. Since such systems are rarely used in most circumstances and at most locations, the bands have been made available for use in addition by, for example, broadband radio communications systems, provided that equipment used in these bands is arranged to detect radar pulses, and to cease transmission on a channel on which radar pulses are detected, so that interference with the operation of the radar may be avoided. In many territories, it is required that equipment be certified to detect specified types of radar pulse transmissions, with a specified range of allowed radar pulse repetition intervals (PRIs) and other specified characteristics such as pulse duration and a threshold for received power in a radar pulse, typically expressed as a received power with respect to an isotropic antenna. If radar pulses with the specified characteristics are detected, a different channel should be selected within a specified time, or if no alternative channel is available, transmission must cease for a specified period; such a process may be may be referred to as Dynamic Frequency Selection (DFS). Typically DFS is required to be implemented in certain frequency bands depending on the territory, and may be required for equipment with a transmit power greater than a specified amount and may be required, for example, for equipment deployed outdoors.
Typically, broadband radio communications equipment for operation in a band where DFS is required must be certified to meet specified requirements regarding probability of detection of radar transmissions. In a broadband radio communication system arranged to transmit and receive in the same frequency channel, such as a system operating according to a contention-based medium access control system, it is typically not possible to detect radar pulses during transmission in the frequency channel. Furthermore, at a terminal of a broadband radio communication system operating, for example, according to a contention-based medium access control system, the timing of the transmission of data packets is typically determined at least in part by the time of arrival, at a data interface, of the data packets to be transmitted. Therefore, it may be possible for data packets to arrive at the data interface spaced apart in time by intervals such that, when transmitted, the transmission has a pulse repetition interval corresponding to an allowed pulse repetition interval of a radar system that the terminal is required to detect. Therefore, the probability of detection of radar pulses may be reduced, according to the arrangement of data packets arriving at the data interface.
It is an object of the invention to mitigate the problems with the prior art systems.